It took me a while to get that one. I first saw it in a joke book when I was a kid and I think I was too literal to understand the humor. Red / Read...they are spelled differently so I just didn't get it. Of course one day, when someone actually said it out loud, I totally got it.

Also in that classic joke book was my favorite joke of all time (very popular on Banana laffy taffy wrappers as well)...

Q: What's the difference between an elephant and a baby aspirin?
A: One is big, grey and rough and the other is little, white and smooth.